The argument to MP_BC_MAKE_FUNCTION (raw code index) was being encoded as a
byte instead of a variable unsigned int. That meant that if there were
more than 128 merged mpy files the encoding would be invalid.
Fix that by using `mp_encode_uint(idx)` to encode the raw code index. And
also use `Opcode` constants for the opcode values to make it easier to
understand the code.
Signed-off-by: Damien George <damien@micropython.org>
bf18ddd989